At least with it being the most basic of transmissions, it is also the most robust, and could take more punishment than other types of transmission. It is the standard system for most cars. The constant mesh transmission moved the problem of connecting two moving parts away from where the gears contacted each other to where the gears contacted the driveshaft which powered the wheels.
Each gear was only loosely connected with the drive shaft, allowing the gear to rotate at a differing speed to the shaft and making gear changes easier. This was accomplished with a device called a dog clutch. Several of these were positioned on the drive shaft between the gears, and were fixed to drive shaft. The loosely mounted gears would already be moving at some speed through their contact with the driveshaft, and the dog clutch moving at full speed would take them the rest of the way as they meshed together resulting in a smoother transition.
Synchromesh transmissions are a further refined version of the constant mesh system, although less common. How it improves the system is by adding another stage to the process of connecting the gears to the drive shaft through the dog clutch.
It splits the dog clutch in two — a gear fixed to the drive shaft called the synchronizer hub, and a collar around the outside of it that could slide back and forth called the shift sleeve.
A new component was added to the gears themselves — the synchroniser cone - and a further moving part called the synchronizer ring was introduced that surrounded the cone. The collars, or shift sleeves, are now the components controlled by the gear stick - and they can slide halfway in either direction onto the synchroniser rings. This forces the rings against the synchroniser cones attached to the gears, and through the increased friction caused by the widening cone it can either speed up or slow down the gear to match the speed of the shift sleeve and synchronizer hub.
Once the speeds are closely enough matched, the sleeve can continue to slide over the blocker ring and directly mesh with both the cone and the gear, connecting everything together and transferring power to the drive shaft.
Unbelievably all of this happens in the fraction of second it takes you to shift gear, providing you with an even smoother gear change. So the next time you seamlessly slide through the ratios, take a moment to appreciate all the thought and work that went into creating the complex arrangement of cogs helping you on your way. You engage reverse gear through a separate, small idler gear. The reverse gear always turns in the opposite direction of the other forward gears.
In years past, double-clutching was common in order to disengage a gear, allow the collar and next gear to reach the same speed, and then to engage the new gear. To double-clutch shift, you pushed the clutch pedal to free the engine from the transmission. Then the collar moved into neutral. You released the clutch and revved the engine to get it to the right rpm value for the next gear so the collar and the next gear spun at the same rate to allow the dog teeth to engage the gear.
When the engine hit the right speed, you depressed the clutch again in order to lock the collar into place on the next gear. Modern cars use synchronizers in order to avoid the need for double-clutching. A synchronizer, or "synchro," lets the collar and gear synchronize their speeds while they're already in contact but before the dog teeth engage. Each manufacturer's synchro is slightly different than the others, but the basic idea is the same. For instance, a cone on one gear will fit into a cone-shaped depression on the collar.
The gear and collar synchronize their speeds thanks to the friction between the cone and collar. Then the outer part of the collar moves out of the way so that the gear can be engaged by the dog teeth. Sign up for our Newsletter!
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